Design

All G.R.A.S. microphone preamplifiers are based on a small ceramic thick-film substrate with a very high input impedance. The ceramic substrate is shielded by a guard ring to minimise the influence of stray capacitance and microphonic interference.The casing is made of stainless steel for maximum strength and durability. The small dimensions of this preamplifier ensure reliable operation under humid conditions owing to the heat generated by internal power dissipation. A 1/2" version is available (G.R.A.S. 26AJ).

Dynamic Range

G.R.A.S- 26AL can handle both single and dual-sided power supplies. The supply can vary between 28 VDC and 120 VDC single-sided or ±14VDC and±60 VDC dual-sided. When using the high supply voltage (120VDCor ±60VDC), the dynamic range exceeds 140 dB.

Noise

The electrical circuit in G.R.A.S. 26AL is built on a ceramic substrate using selected low-noise components to gain very low self-noise. The electrical self-noise is so low that system noise is mainly determined by the microphone capsule’s thermal noise.

Frequency response

The low-frequency cut-off of the G.R.A.S. 26AL preamplifier is mainly determined by the input impedance of the preamplifier and the capacitance of the microphone capsule. The capacities 20 pF, 6.5 pF and 3 pF equal the typical capacitances of ½", ¼" and 1/8" microphone capsules respectively.

The high-frequency cut-off is determined by the preamplifier’s ability to drive capacitive loads (slewrate), caused by the cable. For large-signals, the effects of these parameters must be accounted for when measurements are performed. The large-signal response for Type 26AL for various capacitive loads corresponding to different cable lengths is shown in the data sheet. The output level is in decibels relative to 1 Volt. Typical capacitance for the cable is 100pF/m(30pF/foot).

Connector

Preamplifier G.R.A.S. 26AL (Fig. 1) is provided with a3-m light weight cable terminating in a 7-pin LEMOseries 1B plug. The cable is only 2.5 mm in diameter and will withstand temperatures from-40 oC to +150 oC. An adapter (GR0010) for G.R.A.S. 1/2" microphones is included.